Someone who's dead is not alive anymore. If you accidentally hit a squirrel with your car, you might stop and check to see if it's alive or dead.
People — and animals — who are deceased are dead, and if you forget to water your favorite plant, it may be dead too. You can even use this word for inanimate objects: when your car's battery runs out of power, your car won't start because it's dead. People describe themselves as dead when they're exhausted or lacking appropriate emotion: "Her voice was dead and cold." If your town is boring, you can say, "It's dead here on the weekends."